The present disclosure relates to a process for fabricating gamma TiAl turbine engine components, such as low pressure turbine blades.
FIG. 1 is a schematic diagram of a turbine blade 10 showing the root 12, the airfoil 14, and shroud 16 sections. As can be seen from the figure, the turbine blade 10 has a complex geometry. As a result, it cannot be easily fabricated. A number of process routes, incorporating both cast and wrought processes, have been used to fabricate low pressure turbine (LPT) gamma TiAl blades. For the cast process, investment mold casting is typically used to make oversized blade blanks which are then machined into final blades. The wrought process involves both extrusion and forging prior to machining.
Modern gas turbine engines operate at high speed and require tensile strength far exceeding the strength of cast gamma TiAl. Thus, cast process techniques are disadvantageous.
Recently, a new beta stabilized gamma TiAl alloy, called TNM, has attracted much attention. This alloy has a composition of Ti-43.5Al-4Nb-1Mo-0.2B (all in at %). This alloy solidifies through a beta solidification path which leads to moderate to mild chemical and microstructural segregations. The resultant microstructure consists mainly of lamellar colonies (α2/γ) with gamma and β/B2 phases located primarily at the colony boundaries. Referring now to FIG. 2, there can be seen the tensile strength of TNM vs. an alloy having a composition of Ti-48 at % Al-2 at % Cr-2 at % Nb (known as 48-2-2) and an alloy having a composition of Ti-47 at % Al-2 at % Mn-2 at % Nb-0.8 vol % TiB2 (known as 47XD). Although the tensile strength of the TNM alloy as cast is significantly higher than other alloys used for these parts, it is still below the requirement for a LPT blade application.
Although cast gamma alloys fall short of the tensile strength required for some LPT blade applications, wrought gamma alloys, with proper heat treatments, produce a microstructure that yields a good balance of tensile strength and ductility at room temperature and creep resistance at elevated temperature. For the wrought process, an outer surface of double VAR cast ingots is lightly machined prior to insertion into stainless steel cans for extrusion. The extrusion takes place at a temperature over 2100 degrees Fahrenheit at an extrusion ratio of about 16:1. The extruded bar is then machined to remove the can. The yield of such an extrusion process is about 70%. The machined extruded bar is cut into smaller lengths as blade blanks and then isothermally forged into oversized blades in a molybdenum die at about 2000 degrees Fahrenheit. The yield of this process is about 70%. The oversized blades are heat treated using a two step process to develop a duplex microstructure consisting of globular gamma phase and (α2/γ) lamellar colonies with a small about of β/B2 phase. The root, airfoil, and the shroud sections are machined from these oversized blades to produce the finished blades.
A typical microstructure for the cast-extruded-forged and heat treated material has a duplex microstructure consisting of 47 vol % gamma, 43 vol % of (α2/γ) lamellar colonies and 10 vol. % of β/B2. The tensile properties were measured from the heat treated material at room temperature. The creep property was measured at 700 degrees Centigrade at a pressure of 36 ksi and yielded 1% plastic elongation for 150 hours. The cast-extruded-forged and heat treated material showed a yield stress of 126 ksi, a ultimate tensile stress of 151 ksi, and a 3.3% elongation. Both tensile and creep properties met the design goals for an LPT blade application.
The process steps outlined above however involve significant waste, resulting in unacceptable blade cost. There remains a need for a process which reduces the waste and results in a more acceptable blade cost.